Method and apparatus of slice selective magnetization preparation for moving table MRI

ABSTRACT

The present invention is directed to slice selective magnetization preparation for moving table MRI. The present invention includes a method of magnetization preparation that takes into account patient movement or translation during the imaging process. The present invention adjusts or modifies the frequency at which magnetization preparation pulses are applied to offset the preparation pulse in space. This allows preparation pulses to be interleaved with imaging and enables magnetization preparation without sacrificing other imaging variables or parameters defined for the particular MR study. As such, contrast within a reconstructed image is improved.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of and claims priority of U.S.Ser. No. 10/604,285 filed Jul. 8, 2003 now U.S. Pat. No. 7,251,520, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to MR imaging and, moreparticularly, to a method and apparatus of slice selective magnetizationpreparation for moving table MRI. More specifically, the presentinvention relates to the timing and positioning of preparation RFpulses, e.g. inversion recovery pulses, in a pulse sequence for wholebody axial imaging of a patient being translated through an imagingvolume of an MR system by a moving table.

When a substance such as human tissue is subjected to a uniform magneticfield (polarizing field B₀), the individual magnetic moments of thespins in the tissue attempt to align with this polarizing field, butprecess about it in random order at their characteristic Larmorfrequency. If the substance, or tissue, is subjected to a magnetic field(excitation field B₁) which is in the x-y plane and which is near theLarmor frequency, the net aligned moment, or “longitudinalmagnetization”, M_(Z), may be rotated, or “tipped”, into the x-y planeto produce a net transverse magnetic moment M_(t). A signal is emittedby the excited spins after the excitation signal B₁ is terminated andthis signal may be received and processed to form an image.

When utilizing these signals to produce images, magnetic field gradients(G_(x), G_(y), and G_(z)) are employed. Typically, the region to beimaged is scanned by a sequence of measurement cycles in which thesegradients vary according to the particular localization method beingused. The resulting set of received NMR signals are digitized andprocessed to reconstruct the image using one of many well knownreconstruction techniques.

Moving table MRI is an MR imaging technique that allows for whole bodyimaging in a relatively short amount of time. Ideally, throughcontinuous translation of a patient through an imaging volume, MR datacan be acquired of the chest, abdomen, and pelvis in a singlebreath-hold. Furthermore, it is preferable for a moving table MR studyto provide image quality, contrast, and resolution comparable tostationary or non-moving table studies. Stationary-table studiesfrequently implement contrast-preparation techniques such as inversionrecovery (IR) and saturation recovery to enhance contrast in areconstructed image. These imaging techniques utilize spatiallyselective or spatially and spectrally selective RF pulses at a fixedinterval in time prior to application of imaging RF pulses and readoutgradient pulses. These traditional imaging protocols, however, are notoptimal for moving table MRI.

As suggested by its name, moving table MRI uses a table to translate apatient through an imaging volume during the imaging process. The tablemay incrementally or continuously move the patient through the imagingbore of the MR system. Unlike stationary-table imaging techniques, inmoving table MRI, the patient is translated or moved during dataacquisition. As such, if a preparatory RF pulse is applied at a momentin time to a particular slice or slab of the patient that is fixedrelative to the imaging bore, the tissues subjected to the preparatorypulse will move in the direction of table motion over time. Therefore, aregion or volume of interest that was marked for data acquisition maymove out of the slice or slab and, as such, not present data foracquisition.

A standard IR pulse sequence 2 for obtaining T1-weighted images of apatient positioned on a stationary stable is illustrated graphically inFIG. 4. As is known, the magnetic moments of the spins in tissue areuniformly aligned upon placement of a patient in a uniform B₀ field.This magnetization 3 is then inverted by the application of an RFinversion pulse 4, i.e. 180 degree flip angle, to an imaging volume.Coinciding with the application of the inversion recovery pulse is aslice select gradient 5 that selectively encodes the imaging volume.After the IR pulse 4 is applied, an inversion recovery time TI isobserved that allows for the magnetization 3 to recover and decay inaccordance with T1 and T2 characteristics of the tissue. The longer TI,the more recovery and decay in the magnetization. At the expiration ofTI, another RF pulse 6 as well as slice selective gradient 7 is applied.This RF pulse 6 is generally referenced as an imaging pulse. RF pulse 6is applied to drive the magnetization of the spins in the tissue back tothe transverse plane. Because the RF pulse 6 is preferably applied atTI, those spins that had a zero magnetization are nulled. Asillustrated, for T1-weighted imaging, an imaging module 8, i.e. phaseencoding and frequency encoding gradients, is applied relativelyimmediately after RF pulse 6 to acquire MR data from the non-nulledmagnetization.

When applied to moving table MRI it is clear that the spins to which theIR pulse was directed, will not only recover toward equilibriumlongitudinal magnetization during TI but will move in the direction oftable motion. Depending upon the slice/slab thickness set for theparticular imaging session and the velocity of table translation, thespins may no longer be in the slice, slab, or volume of interest when TIexpires and, as such, not detected during readout.

One possible solution is to adjust the timing of the imaging RF pulserelative to the preparation RF pulse so that the TI period is changed.However, the user selects the appropriate TI period typically as afunction of the T1 values of the targeted tissue. Since a preparation RFpulse, such as an inversion RF pulse, causes magnetization to be drivenbelow the transverse plane, the magnetization will progress towardpositive magnetization from the inverse or negative magnetization. Assuch, adjusting TI will not achieve the effect sought with preparationRF pulses and, as such, affects image contrast as well as imageintensity; both of which could negatively affect the diagnostic value ofthe reconstructed image.

Another proposed solution is to acquire imaging data during TI, i.e. asmagnetization recovers. However, if data from the imaging volume iscollected during the approach from inversion to fully recoveredmagnetization image contrast will change on a slice-by-slice basis.Variations in image contrast on a per slice basis also negatively affectthe diagnostic value of the reconstructed image.

It would therefore be desirable to have a system and method capable ofachieving slice selective magnetization preparation for moving tableMRI.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a system and method of slice selectivemagnetization preparation for imaging of a patient being translatedthrough an imaging volume by a moving table overcoming theaforementioned drawbacks.

Moving table MRI allows for a whole body exam to be completed in arelatively short period of time. The present invention is directed toslice selective magnetization preparation for moving table MRI. Thepresent invention, which is well suited for gradient echo or spin echoimaging techniques, includes a method of magnetization preparation thattakes into account patient movement or translation during the imagingprocess. In this regard, the present invention adjusts or modifies thefrequency at which magnetization preparation pulses are applied withoutsacrificing other imaging variables or parameters defined for theparticular MR study. As such, contrast within a reconstructed image isimproved.

Therefore, in accordance with one aspect of the present invention, amethod of slice selective magnetization for moving table MRI comprisesthe steps of defining a fixed imaging slab. The process further includesthe step of applying a preparation RF pulse to prepare a region ofinterest outside the fixed imaging slab. The prepared region of interestis then translated to the fixed imaging slab whereupon an imaging RFpulse is applied to the fixed imaging slab to acquire MR data of theprepared region of interest.

In accordance with another aspect of the invention, an MRI apparatusincludes an MRI system having a plurality of gradient coils positionedabout a bore of a magnet to impress a polarizing magnetic field andspatially encode spins. An RF transceiver system and an RF switch arecontrolled by a pulse module to transmit and receive RF signals to andfrom an RF coil assembly to acquire MR images. The MRI apparatus alsoincludes a computer programmed to receive a user input identifying apreparation interval (e.g. TI) for a pulse sequence to acquire data of asubject being continuously translated through an imaging volume. Fromthe preparation interval, the computer is programmed to determine anoffset value, f_(off), to be applied to a preparation RF pulse of thepulse sequence to modify application of the preparation RF pulse toaccount for translation of the subject through the imaging volume. Thecomputer is also programmed to generate a modified pulse sequence suchthat application of the preparation RF pulse has been modified by theoffset value.

In accordance with another aspect of the invention, the invention isembodied in a computer program stored on a computer readable storagemedium and having instructions which, when executed by a computer, causethe computer to determine a distance spins of a prepared tissue of apatient will travel while the patient is translated through an imagingvolume by a moving table. The computer is also caused to determine, fromthe distance, a preparation volume of interest. The instructions alsocause the computer to generate an imaging sequence to acquire data froma patient being translated past a fixed imaging volume such that thepreparation volume of interest is prepared before being presented in thefixed imaging volume.

Various other features, objects and advantages of the present inventionwill be made apparent from the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate one preferred embodiment presently contemplatedfor carrying out the invention.

In the drawings:

FIG. 1 is a schematic block diagram of an MR imaging system for use withthe present invention.

FIG. 2 is a schematic of a portion of a pulse sequence in accordancewith the present invention.

FIG. 3 is a flow chart setting forth an imaging pulse in accordance withthe present invention.

FIG. 4 is a schematic of a known inversion recovery pulse sequence.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A system is shown to prepare magnetization of a slice, slab, or volumefixed in position relative to a patient being translated continuouslythrough an imaging volume. One skilled in the art will appreciate thepresent invention is applicable with single slice preparation,multi-slice or slab preparation, or volume preparation. As such, theseterms may be used interchangeably throughout the application. Thepresent invention will be described with respect to an inversionrecovery, gradient echo pulse sequence; however, the present inventionis equivalently applicable with other types of preparation RF pulses, aswell as spin echo sequences.

Referring to FIG. 1, the major components of a preferred magneticresonance imaging (MRI) system 10 incorporating the present inventionare shown. The operation of the system is controlled from an operatorconsole 12 which includes a keyboard or other input device 13, a controlpanel 14, and a display screen 16. The console 12 communicates through alink 18 with a separate computer system 20 that enables an operator tocontrol the production and display of images on the display screen 16.The computer system 20 includes a number of modules which communicatewith each other through a backplane 20 a. These include an imageprocessor module 22, a CPU module 24 and a memory module 26, known inthe art as a frame buffer for storing image data arrays. The computersystem 20 is linked to disk storage 28 and tape drive 30 for storage ofimage data and programs, and communicates with a separate system control32 through a high speed serial link 34. The input device 13 can includea mouse, joystick, keyboard, track ball, touch activated screen, lightwand, voice control, or any similar or equivalent input device, and maybe used for interactive geometry prescription.

The system control 32 includes a set of modules connected together by abackplane 32 a. These include a CPU module 36 and a pulse generatormodule 38 which connects to the operator console 12 through a seriallink 40. It is through link 40 that the system control 32 receivescommands from the operator to indicate the scan sequence that is to beperformed. The pulse generator module 38 operates the system componentsto carry out the desired scan sequence and produces data which indicatesthe timing, strength and shape of the RF pulses produced, and the timingand length of the data acquisition window. The pulse generator module 38connects to a set of gradient amplifiers 42, to indicate the timing andshape of the gradient pulses that are produced during the scan. Thepulse generator module 38 can also receive patient data from aphysiological acquisition controller 44 that receives signals from anumber of different sensors connected to the patient, such as ECGsignals from electrodes attached to the patient. And finally, the pulsegenerator module 38 connects to a scan room interface circuit 46 whichreceives signals from various sensors associated with the condition ofthe patient and the magnet system. It is also through the scan roominterface circuit 46 that a patient positioning system 48 receivescommands to move the patient to the desired position for the scan.

The gradient waveforms produced by the pulse generator module 38 areapplied to the gradient amplifier system 42 having G_(x), G_(y), andG_(z) amplifiers. Each gradient amplifier excites a correspondingphysical gradient coil in a gradient coil assembly generally designated50 to produce the magnetic field gradients used for spatially encodingacquired signals. The gradient coil assembly 50 forms part of a magnetassembly 52 which includes a polarizing magnet 54 and a whole-body RFcoil 56. A transceiver module 58 in the system control 32 producespulses which are amplified by an RF amplifier 60 and coupled to the RFcoil 56 by a transmit/receive switch 62. The resulting signals emittedby the excited nuclei in the patient may be sensed by the same RF coil56 and coupled through the transmit/receive switch 62 to a preamplifier64. The amplified MR signals are demodulated, filtered, and digitized inthe receiver section of the transceiver 58. The transmit/receive switch62 is controlled by a signal from the pulse generator module 38 toelectrically connect the RF amplifier 60 to the coil 56 during thetransmit mode and to connect the preamplifier 64 to the coil 56 duringthe receive mode. The transmit/receive switch 62 can also enable aseparate RF coil (for example, a surface coil) to be used in either thetransmit or receive mode.

The MR signals picked up by the RF coil 56 are digitized by thetransceiver module 58 and transferred to a memory module 66 in thesystem control 32. A scan is complete when an array of raw k-space datahas been acquired in the memory module 66. This raw k-space data isrearranged into separate k-space data arrays for each image to bereconstructed, and each of these is input to an array processor 68 whichoperates to Fourier transform the data into an array of image data. Thisimage data is conveyed through the serial link 34 to the computer system20 where it is stored in memory, such as disk storage 28. In response tocommands received from the operator console 12, this image data may bearchived in long term storage, such as on the tape drive 30, or it maybe further processed by the image processor 22 and conveyed to theoperator console 12 and presented on the display 16.

The present invention is directed at a method and system ofmagnetization preparation for moving table MRI. While particularlyapplicable for whole body axial imaging, the invention is equivalentlyapplicable for other imaging protocols. Additionally, the presentinvention will be described with respect to continuous translation ofthe patient, but may be applicable with incremental patient translation.Further, while the present invention will be described for the specificexample of an inversion recovery preparation sequence, it isequivalently applicable for other types of magnetization preparation.

It is generally understood that the preparation RF pulses and theimaging excitation RF pulses of a pulse sequence excite the same sliceor volume relative to the magnet of the MR system. That is, for movingtable MRI, a slice or slab that remains fixed relative to the MR magnetis defined along an imaging axis, i.e. z-axis. As such, as the patientis translated or moved along the imaging axis, different anatomicalregions will pass through the slice or slab of interest at differentpoints in time. This allows, for instance, imaging of the pelvis,abdomen, and chest regions of the patient in a single breath-hold. Aspointed out previously; however, the tissue or region of interest (orportion thereof) may pass through the fixed slice or slab after apreparation RF pulse is applied but before data acquisition therebyresulting in the region of interest not being completely, if at all,imaged.

Therefore, in accordance with the present invention, and referring toFIG. 2, a diagram showing the spatial relationship of the slicesselected by the magnetization preparation and imaging RF pulses 70 formagnetization preparation for moving table MRI is illustrated. Similarto standard imaging sequences, diagram 70 includes an IR or preparationmodule 72 and a main or imaging module 74. The IR module 72 representsapplication of a preparation RF pulse, i.e., an inversion recovery pulsehaving a flip angle of 180 degrees, as well as spoiler gradients todephase any magnetization inadvertently tipped into the transverseplane. The preparation RF pulse may be a spatially selective, or aspatially and spectrally selective RF pulse. Module 74 represents theimaging RF pulse as well as the encoding and readout gradients thatfollow. It should be noted that the present invention is applicable withgradient as well as spin echo imaging techniques including rapid imagingvariations such as echo-planar imaging, spiral imaging, projectionreconstruction, and fast spin echo.

As shown in FIG. 2, the IR module 72 occurs before imaging module 74 inboth space and time. That is, as the table moves the patient along atranslation direction, the pulses of the IR module are first applied toinvert a volume of interest different from an imaging volume (both withrespect to the MR magnet) and are followed by application of the pulsesof the imaging module at some predefined period of time, TI, thereafter.Since the patient is being moved parallel to table motion direction, thespins in the tissue of the patient will be prepared, i.e. inverted, at apoint in time and space before those spins reach the imaging volume. Aswill be described below, the spins will be subjected to the preparationRF pulse at a moment that is a function of recovery time, TI. Thevelocity by which the table moves the patient passed the fixed slice orslab and the recovery time, TI, determine the distance separating thepreparation volume from the imaging volume.

As will be described in greater detail below, the IR module is appliedto a set of spins of a tissue that is not in the imaging volume fixedrelative to the MR magnet when the IR pulse is applied. That is, the IRmodule is applied to an inverted volume different from the imagingvolume. Since the patient is being continuously translated through thebore of the MR magnet, MR data may be acquired of the imaging volumewhile the spins of the inverted volume recover. As such, the spins ofthe imaging volume may be prepared prior to reaching the imaging volumethat remains fixed relative to the MR magnet. This allows for nearlycontinuous data acquisition of the patient as it translates through theimaging volume while simultaneously allowing for preparation of asoon-to-be imaged tissue or inverted volume, i.e. prior to thesoon-to-be imaged tissue entering the imaging volume. The preparationsequence can be temporally interleaved with the imaging sequence. Inthis regard, data acquisition of the imaging volume is minimallyaffected by the inversion recovery time, TI.

In continuously moving table MRI, the distance, D_(ist), a particularspin will travel within a single recovery time, TI, can be determinedbased on table or patient velocity, ν, and the value set for TI by theuser when prescribing the scan study. That is, the distance traveled bya spin following application of an IR pulse is:D _(ist) =ν·TI  (Eqn. 1).

From the product of the recovery time and the velocity of table motion,a pulse frequency offset value, f_(off), can be determined in accordancewith the following:f _(off) =γ·A _(ss) ·D _(ist)  (Eqn. 2),where γ represents the gyromagnetic ratio of 42.58×10⁶ Hz/T and A_(ss)represents the amplitude of the slice selective gradient applied duringthe preparation RF pulse. The pulse frequency offset value is applied tothe standard frequency of the preparation RF pulse to be applied in theabsence of table movement. For example, if the operator selects a TI of100 ms and the table velocity of 1.25 cm/sec then the D_(ist) would be12.5 mm leading to a particular frequency offset value for the IRpreparation RF pulse. The application of the inversion pulse would be TItime earlier than the acquisition of the corresponding data set. Itshould be noted that the inversion pulse sequence is interleaved withthe imaging pulse sequence and as the acquisition of the appropriatedata set starts the prepared volume enters the imaging volume courtesyof the table movement. Since the offset is to be applied in a directionthat is opposite of table motion, this allows the inverted volume torecover in time as it moves in space to the imaging volume defined bythe fixed slice or slab relative to the MR magnet. The TI period remainsthe same. It should be noted that the size of the optimal volume of theMR scanner limits the maximum distance between the volumes selected bythe preparation pulse and the imaging pulse. Therefore, D_(ist) shouldbe smaller than the size of the optimal imaging volume in the directionof table motion. It should also be noted that since the preparation RFpulse is interleaved with the imaging pulse, there is nearly continuousacquisition of data leading to efficient collection of the data whileachieving the desired contrast.

Applying the preparation RF pulse at the offset frequency value allowsfor the spins that are the subject of data acquisition to not onlyrecover from the inverted magnetization during TI, but also allows forthe spins to be translated during that same TI period in space such thatthe prepared magnetization are presented at the time period for dataacquisition. In this regard, in one embodiment, the preparation RFpulses may be applied or played out with the same slice thickness as theimaging RF pulses and are repeated at an interval that coincides withthe amount of time necessary for the table to translate the patient oneslice thickness.

However, one skilled in the art will appreciate that the repetitioninterval of the preparation pulses, i.e., time between application ofpreparation RF pulses, could be set to a value different from thatneeded for the table to move the patient one slice thickness. Further,the slice thicknesses of the preparation pulse and the imaging RF pulsesmay be different and this could impact the repetition interval.Additionally, the repetition interval of the preparation pulse can bechanged as desired for a particular MR study. To further enhancecontrast differences as well as the robustness of the moving table MRI,the type of preparation pulse could be varied as the table is moved toachieve different contrast properties or different image quality. In yeta further embodiment, the offset could be varied, depending on the goalsof a particular study, on a per TR basis. As such, different parts ofthe anatomy could be imaged differently. In this regard, the presentinvention not only improves the contrast and effectiveness of movingtable MRI, it improves the flexibility and quality of image contrast.

Referring now to FIG. 3, the steps of an imaging process implementingthe present invention is shown. The process 100 begins at 102 with apatient being positioned on a table for MR imaging. Shortly thereafter,an MR technologist or radiologist inputs at step 104 a series of imagingparameter identifiers that could include TR, T1 and T2 values of atargeted tissue, type of imaging sequence, type of preparatory pulses tobe applied, TI, flip angles, table velocity, slice thickness, k-spacedimensions, and the like. From the table velocity and TI values entered(or analogous parameter for a different type of contrast preparation),the distance a spin will travel during the TI at the identified tablevelocity is determined at step 106 in accordance with Equation 1. Fromthe distance determined by Equation 1, the frequency offset to apply inaccordance with Equation 2 is determined at 108. The repetition intervaland timing of the preparation pulse is the determined at 110. Inaccordance with the user inputs at step 104, the offset of themagnetization preparation pulse at 108, and the timing and repetitioninterval of the preparation RF pulses at step 110, a pulse sequencetailored to the imaging parameters identified at step 104 is generatedat 112. The pulse sequence may then be applied at step 114 as thepatient is translated continuously through the imaging volume for dataacquisition.

Therefore, in accordance with one embodiment of the present invention, amethod of slice selective magnetization preparation for moving table MRIcomprises the steps of defining a fixed imaging slab. The processfurther includes the step of applying a preparation RF pulse to preparea region of interest outside the fixed imaging slab. The prepared regionof interest is then translated to the fixed imaging slab whereupon animaging RF pulse is applied to the fixed imaging slab to acquire MR dataof the prepared region of interest.

In accordance with another embodiment of the invention, an MRI apparatusincludes an MRI system having a plurality of gradient coils positionedabout a bore of a magnet to impress a polarizing magnetic field andspatially encode spins. An RF transceiver system and an RF switch arecontrolled by a pulse module to transmit and receive RF signals to andfrom an RF coil assembly to acquire MR images. The MRI apparatus alsoincludes a computer programmed to receive a user input identifying apreparation interval (e.g. TI) for a pulse sequence to acquire data of asubject being continuously translated through an imaging volume. Fromthe preparation interval, the computer is programmed to determine anoffset value, f_(off), to be applied to a preparation RF pulse of thepulse sequence to modify application of the preparation RF pulse toaccount for translation of the subject through the imaging volume. Thecomputer is also programmed to generate a modified pulse sequence suchthat application of the preparation RF pulse has been modified by theoffset value.

In accordance with another embodiment of the invention, the invention isembodied in a computer program stored on a computer readable storagemedium and having instructions which, when executed by a computer, causethe computer to determine a distance spins of a patient will travelwhile the patient is translated through an imaging volume by a movingtable. The computer is also caused to determine, from the distance, apreparation volume of interest. The instructions also cause the computerto generate an imaging sequence to acquire data from a patient beingtranslated past a fixed imaging volume such that the preparation volumeof interest is prepared before being presented in the fixed imagingvolume.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A method of slice selective magnetization preparation for movingtable MRI, the method comprising the steps of: defining a fixed imagingslice within an optimal imaging volume of an MR system; applying apreparation RF pulse to prepare a first region of interest of a subjectto be imaged, the first region of interest located outside the fixedimaging slice but within the optimal imaging volume; translating thesubject on a moving table such that the prepared first region ofinterest is translated to the fixed imaging slice; and; applying animaging RF pulse to the fixed imaging slice to acquire MR data of theprepared first region of interest.
 2. The method of claim 1 wherein thefirst region of interest has a same width as that of the fixed imagingslice.
 3. The method of claim 1 further comprising the step of applyinga preparation RF pulse to a second region of interest different locatedoutside the fixed imaging slice but within the optimal imaging volumeand acquiring MR data from the first region of interest present at thefixed imaging slice.
 4. The method of claim 1 further comprising thestep of offsetting application of the preparation RF pulse as a functionof translation direction.
 5. The method of claim 4 further comprisingthe step of offsetting application of the preparation RF pulse oppositeto translation direction.
 6. The method of claim 1 further comprisingthe step of determining an offset value from a product of translationdistance of a spin at a user-defined TI, amplitude of a slice selectivegradient applied to the imaging volume, and a gyromagnetic ratio.
 7. Themethod of claim 1 further comprising the step of continuouslytranslating the subject on the moving table through the imaging volumeduring application of the preparation pulse and during acquisition of MRdata.
 8. The method of claim 1 wherein the preparation RF pulse includesan inversion recovery pulse.
 9. The method of claim 1 further comprisingthe step of applying the preparation RF pulse such that a time betweenapplication of preparation RF pulses is substantially equivalent to anamount of time needed to translate the subject on the moving table oneimaging slice thickness.
 10. An MRI apparatus comprising: a magneticresonance imaging (MRI) system having a plurality of gradient coilspositioned about a bore of a magnet to spatially encode spins and an RFtransceiver system and an RF switch controlled by a pulse module totransmit RF signals to an RF coil assembly to acquire MR images; and acomputer programmed to: receive a user input identifying a preparationinterval, TI, for a pulse sequence that acquires data of a patient beingcontinuously translated through an imaging volume via a moving table;from the preparation interval, determine a frequency offset value,f_(off), to be applied to a preparation RF pulse of the pulse sequence;modify application of the preparation RF pulse to account fortranslation of the patient through the imaging volume; and generate amodified pulse sequence such that the preparation RF pulse has beenmodified by the offset value.
 11. The MRI apparatus of claim 10 whereinthe offset value is defined by:f _(off) =γ*A _(ss) *ν*TI, where γ is a gyromagnetic ratio, A_(ss) is anamplitude of a slice selective gradient applied during the preparationRF pulse, ν is a velocity of the patient being translated on the movingtable, and TI is a preparation interval.
 12. The MRI apparatus of claim10 wherein the computer is further programmed to determine a directionof patient translation and apply the offset value to the preparation RFpulse opposite to the direction of patient translation.
 13. The MRIapparatus of claim 10 wherein the modified preparation RF pulse includesan inversion recovery pulse.
 14. The MRI apparatus of claim 10 whereinthe computer is further programmed to repeat application of eachpreparation RF pulse at a time substantially equivalent to that neededto translate the patient on the moving table one slice thickness. 15.The MRI apparatus of claim 10 wherein the modified pulse sequenceincludes a gradient echo sequence.
 16. The MRI apparatus of claim 10wherein the computer is further programmed to change at least one of thepreparation interval and the preparation sequence in a prescribed manneras the patient is translated on the moving table.
 17. A computerreadable storage medium having a computer program stored thereon toacquire MR data of a patient being translated through an imaging volume,the computer program having a set of instructions that when executedcauses a computer to: determine a distance that spins of a magnetizationprepared tissue of a patient will travel while the patient is translatedthrough an imaging volume by a moving table during a prescribedpreparation interval defined as the time between application of apreparation pulse and commencement of an imaging pulse sequence;determine, from the distance, a preparation volume of interest of thepatient; and generate the imaging pulse sequence to acquire MR data fromthe patient being translated through a fixed imaging volume such thatthe preparation volume of interest is prepared before being presented inthe fixed imaging volume.
 18. The computer readable storage medium ofclaim 17 wherein the set of instructions further causes the computer todetermine an amplitude of a slice selective gradient and multiply theamplitude and a gyromagnetic ratio to the distance to determine thefrequency offset of the preparation pulse.
 19. The computer readablestorage medium of claim 17 wherein the computer is further programmed todefine the preparation volume of interest to have the same slicethickness as the fixed imaging volume.
 20. The computer readable storagemedium of claim 17 wherein the imaging volume includes one of a singleslice and a slab of multiple slices.